In Patent Literature 1, a laser machining apparatus including a reflective spatial light modulator is described. This laser machining apparatus includes a laser light source and a spatial light modulator, and a polarization beam splitter is arranged on an optical path between the laser light source and the spatial light modulator. Further, a Faraday rotator is arranged on an optical path between the polarization beam splitter and the spatial light modulator.
In this laser machining apparatus, light output from the laser light source is transmitted through the polarization beam splitter and reaches the spatial light modulator via the Faraday rotator, and a phase or amplitude of the light is modulated. Also, the light after the modulation passes through the Faraday rotator and reaches the polarization beam splitter again. Here, the Faraday rotator rotates a polarization plane 45° in a forward path, and rotates the polarization plane 45° in the same rotation direction in a backward path. Accordingly, the light reaching the polarization beam splitter again is reflected by the polarization beam splitter since the polarization plane thereof is rotated 90°.
In Patent Literature 2, a phase modulation system including a reflective phase modulator is described. This phase modulation system includes a polarization beam splitter arranged on an optical path of light input on the phase modulator, and a Faraday rotator arranged between the phase modulator and the polarization beam splitter. In this phase modulation system, light is modulated through the same operation as that in the laser machining apparatus described in Patent Literature 1.